Defocus or out-of-focus is a word corresponding to focus. It means that the image plane is not in focus, and is divided into two states—front defocus (in front of the focus) and back defocus (behind the focus).
The main reason for increase in the degree of myopia is the increase in the axial length of the eye. For every 1 mm increase in the axial length of the human eye, myopia grows by 3.00 degrees. Recent medical studies have proved that the extension of the eyeball depends on the defocus at the periphery of the retina (as shown by 10 in FIG. 1). According to dioptric concepts, where the focus falls in front of the retina, it is called myopic defocus (as shown by 30 in FIG. 1); where the focus falls behind the retina, it is called hyperopic defocus (as shown by 20 in FIG. 1). A myopic eye exhibits myopic defocus in the center of the retina, but hyperopic defocus in the periphery of the retina. Hyperopic defocus in the periphery of the retina is the main reason for constant increase in the degree of myopia.
The eyeball has the characteristic of relying on imaging in the periphery of the retina to induce the eyeball development, especially for adolescents under the age of 18. If the peripheral retinal imaging is hyperopically out of focus, the retina tends to grow toward the image point and the eyeball length will increase. If the peripheral retinal imaging is myopically out of focus, the eyeball will stop extending. If, through modern medical methods, hyperopic defocus in the periphery of retina is corrected or myopic defocus is formed artificially in the periphery of retina, constant increase in the degree of myopia can be inhibited. Besides, the occurrence and progress of myopia can be prevented effectively by finding out the causes of defocus in the periphery of the retina.
The concept of peripheral defocus is derived from clinical practice in the field of optometry. Doctors at first found that the axial length and myopia growth of the eyes of some of the orthokeratology lens wearers was retarded, and then discovered the role of peripheral defocus in this process and formed the theory that peripheral defocus controls myopia. However, this theory has been in a state of passive discovery. The discussions among doctors and researchers in the field remain at the level of huge statistics and analysis of the peripheral defocus of the human eye, without forming an effective and quantifiable treatment implementation plan. Enterprises, research institutes and the like stay at the level of proposing some preliminary products with uncontrollable degree of peripheral defocus, such as frame glasses of a partitioned structure and optical defocus soft contact lenses, besides orthokeratology lenses which appear earlier.
The mechanism of controlling the peripheral defocus of the orthokeratology lens is to shape the anterior surface of the cornea into the shape of the inner surface of the optical zone of the orthokeratology lens (spheric surface) by wearing the lens at night, taking advantage of the activity of the cells at the surface of the cornea, and thus form of hyperopic peripheral defocus.
The disadvantage of the orthokeratology lens is that the curvature of the retina varies from one patient to another. The existing orthokeratology lens shapes the outer surface of the cornea into the spherical shape of its base curve zone, and the refractive power distribution of the outer surface of the cornea only follows the rules of refractive power distribution of the spherical surface. That is, for the same radius of curvature of the anterior surface of the shaped cornea, the refractive power distribution thereof has only a single form. When the curvature of the retina of the human eye is greater than the curvature of the refractive power distribution formed by the cornea, myopic peripheral defocus cannot be formed, and thus myopia growth cannot be controlled. Therefore, the orthokeratology lens whose base curve zone is a spherical surface cannot achieve controllable, effective peripheral refractive power control. It benefits only some of the patients and controls their myopia growth, but cannot achieve effective control of myopia of every patient.
Frame glasses have a partitioned structure. The center of the glass is designed as a precise imaging 0 spherical aberration optical zone, and the edge as a peripheral defocus control zone with a higher refractive power than the central region. The problem with this approach is that peripheral defocus exists only outside the often used optical zone, and does not work in most cases. The myopia control zone is very limited and not continuous.
As to optical defocus soft contact lenses, the lens surface structure is divided into multiple layers, which are designed to have different radians (radius of curvature). Two radians alternate to achieve hyperopic defocus of the refractive power. However, there are two problems with this way of realizing peripheral defocus control. Firstly, since the lens has only two radians, the optical imaging process is similar to that of a partitioned multifocal lens. The focuses interfere with each other and form a halo. Secondly, since the radii of curvature of the curve segments are different, joining of the rings would cause a large amount of stray light. Therefore, the biggest problem with this kind of lens is that imaging is disturbed by the multi-layer structure of the optical zone and the visual quality is poor.
So far, the technique of control myopia growth through peripheral defocus faces two major problems—a lack of a clear and quantifiable peripheral defocus control implementation plan, and a lack of an effective, controllable therapeutic product.
Therefore, there is a particular need for a method for preparing an aspheric vision correction lens with controllable peripheral defocus which can provide, according to the patient's own physiological and refractive state, a custom quantitative peripheral defocus product with a controllable degree of defocus to solve the aforesaid existing problem.
There are two types of vision correction lenses worn outside the eye—lenses in direct contact with the human eye (such as cornea contact lenses) and lenses that do not directly contact the human eye (such as frame glasses). Frame glasses are generally made of glass or resin, and have a refractive index of about 1.40 to 1.71. A cornea contact lens is one worn on the cornea of the eyeball to correct vision or protect the eye. There are three types of cornea contact lenses—rigid, semi-rigid, and soft ones, depending on the hardness of the material. The refractive index is about 1.40 to 1.50.
In the prior art, optical defocus soft contact lens is a peripheral defocus control type cornea contact lens. The surface structure of the lens is divided into multiple layers designed to have different radians (radius of curvature). Two radians alternate to realize myopic peripheral defocus of the refractive power. There are two problems with this way of realizing peripheral defocus control. Firstly, since the lens has only two radians, the optical imaging process is similar to that of a partitioned multifocal lens. The focuses interfere with each other and form a halo. Secondly, since the radii of curvature of the curve segments are different, joining of the rings would cause a large amount of stray light. Therefore, the biggest problem with this kind of lens is that imaging is disturbed by the multi-layer structure of the optical zone and the visual quality is poor.
Existing frame glasses have a partitioned structure. The center of the glass is designed as a precise imaging 0 spherical aberration optical zone, and the edge as a peripheral defocus control zone with a higher refractive power than the central region. The problem with this approach is that peripheral defocus exists only outside the often used optical zone, and does not work in most cases. The myopia control zone is very limited and not continuous.
Therefore, there is a particular need for a vision correction lens worn outside the eye to solve the aforesaid existing problem.
The design principle of “reverse geometry” is used for orthokeratology lenses. The surface (inner surface) of the entire lens contacting the cornea is designed as several curve segments joined to each other. When the lens is worn, the special shape of the inner surface of the lens causes a layer of unevenly distributed tears between the lens and the outer surface of the cornea. The hydrodynamic effect of the tears pulls the epithelial cells at the center of the cornea to the mid-peripheral portion (periphery); meanwhile, when the eye is closed, the eyelid causes the center of the lens to apply a certain pressure to the lower cornea. These two effects lead to the flattening of the curvature of the center of the cornea, and the corneal shape tends to be the shape of the base curve zone of the inner surface of the orthokeratology lens. After the lens is taken off, the refractive state of the human eye changes. The visual imaging point moves closer to the retina, thereby correcting myopia.
The “reverse geometry” design of the orthokeratology lens was proposed by Stoyan in 1989 (U.S. Pat. No. 4,952,045). The original reverse geometry design divided the orthokeratology lens into three curve zones—base curve zone, reverse curve zone and peripheral curve zone. Since the reverse curve zone of this design is very wide, the height of edge lift is large, which tends to cause irregular movement of the lens. This design has great limitations in clinical application.
Orthokeratology lenses of modern “reverse geometry” design have modified the reverse geometry zone, and are generally divided into four zones. As shown in FIG. 12, the base curve zone 11 is in contact with the central region of the cornea and is relatively flat in shape for flattening the surface of the cornea. The reverse curve zone 12 is relatively steep for reinforcing the flattening effect of the base curve zone 11 and ensuring a certain amount of tear storage. The alignment curve zone 13, also called fitting curve zone is mainly for stabilizing the lens. The peripheral curve zone 14 ensures the circulation of tears between the cornea and the periphery of the orthokeratology lens.
The inner surface of the orthokeratology lens is the region where the shaping function is realized, and most of the design is done in this region. The region is designed based on two variables—radius of curvature and width of the four curve zones, according to the patient's corneal shape and desired diopter.
At present, the design widely used in production generally has 4 to 7 or 5 to 7 curves of different radii of curvature and joined together. As shown in FIG. 12, four curve zones are the most basic design. The four curve zones take the form of four spherical surfaces with different radii of curvature, and are chamfered at the joining thereof so that the curve zones are joined naturally. 5 to 7 curves joined together means that a plurality of curves are used in the reverse curve zone 12 and alignment curve zone 13 (e.g., two curves are used in the reverse curve zone, and three curves are used in the alignment curve zone) so that the base curve zone 11 and the reverse curve zone 12 are joined more easily and the alignment curve zone 13 better fits the corneal shape (since the cornea is aspheric, a plurality of spherical surfaces are used to fit the aspheric shape). In the prior art there are also designs that use an aspheric alignment curve.
Due to the activity of corneal cells, change of the shape of the cornea brought by the orthokeratology lens is only temporary. When the patient stops wearing the orthokeratology lens, the cornea will return to its original shape. Therefore, the original orthokeratology lens is considered only as a treatment means for temporary correction of myopia. However, clinical research in subsequent years found that wearing orthokeratology lens can slow down the increase of the axial length of the human eye for some adolescents, and thus control the development of myopia. Clinical research indicates that the formation of myopic peripheral defocus after wearing the orthokeratology lens is the mechanism based on which the orthokeratology lens works.
The cornea of a normal human eye is generally aspheric, the periphery being slightly flatter than the center. After the corneal shaping, the anterior surface of the cornea becomes spherical, namely takes the shape of the posterior surface of the orthokeratology lens. FIG. 13 is a schematic diagram of the variation of the refractive power of a spherical cornea (as shown in by A in the figure) and an aspheric cornea (as shown by B in the figure) of the same radius of curvature, along with the aperture. It can be seen that compared with the aspheric cornea, the spherical cornea brings a greater refractive power to the periphery of the human eye. Therefore, the true mechanism of the orthokeratology lens controlling myopia growth is that while being worn at night, the orthokeratology lens shapes the cornea into a spherical surface (the shape of the inner surface of the optical zone of the orthokeratology lens), so that when seeing an object, the human eye has a greater refractive power in the periphery than before, enabling some wearers to form myopic peripheral defocus and thereby slowing the increase in the axial length of the human eye and controlling the development of myopia.
The base curve zones of the existing orthokeratology lenses all have a spherical surface. Spherical base curve zone will shape the anterior surface of the cornea into a spherical surface, so the refractive power distribution provided by the cornea is in line with spherical characteristics. The disadvantage is that the curvature of the retina varies from patient to patient. The existing orthokeratology lens shapes the outer surface of the cornea into the spherical shape of its base curve zone, and the refractive power distribution of the cornea only complies with the refractive power distribution rules of the spherical surface. That is, for the same radius of curvature of the anterior surface of the shaped cornea, the refractive power distribution of the cornea has only one form. For example, for a shaped cornea having a radius of curvature of 42.25 D, its refractive power distribution can only be the case as shown by A in FIG. 13. When the curvature of the human eye retina is greater than the curvature of the refractive power distribution formed by the cornea as shown in the figure, myopic peripheral defocus cannot be formed, and myopia growth cannot be controlled. Therefore, the orthokeratology lens whose base curve has a spherical surface cannot form controllable, effective peripheral refractive power control. Therefore, it benefits only some of the patients and controls their myopia growth, but cannot achieve effective control of myopia of every patient.
Some of the existing orthokeratology lenses use an aspheric design. For example, Berke in U.S. Pat. No. 7,984,988 B2 designs the base curve zone of the orthokeratology lens as an ellipsoid; Sami G. EI Hage in U.S. Pat. No. 5,695,509 suggests determining key coordinate points according to the corneal shape and tear thickness, achieving aspheric fitting using the coordinate points, and determining the shape of the inner surface of the orthokeratology len. Patent 201420052256.2 designs the anterior surface of the orthokeratology lens as an aspheric surface to prevent the human eye from the interference of spherical aberration at night when wearing it so as to improve visual quality. The goals of these designs are all for the human eye to have better visual quality after shaping the cornea. The refractive power distribution of the entire eye is made to be as consistent at all apertures as possible, leading to hyperopic peripheral defocus. This is contrary to the purpose and method of controlling myopia through peripheral defocus.
Therefore, there is a particular need for an orthokeratology lens whose base curve zone is a special aspheric surface to achieve controllable myopic peripheral defocus to solve the aforesaid existing problem.
Intraocular lens mainly refers to a phakic intraocular lens (PIOL) for myopia refraction, PIOL is a negative-power lens implanted surgically between the cornea and lens of the human eye to correct refractive error of the human eye.
PIOLs are divided into anterior chamber type and posterior chamber type according to the implantation position. The posterior surface of the anterior chamber type is generally relatively flat and the anterior surface plays a major role in refraction. The anterior surface of the posterior chamber type is generally relatively flat, and the posterior surface plays a major role in refraction.
Existing PIOLs on the market use a spherical design. Patent 201520014249.8 discloses an aspheric PIOL, which aims to maintain the total refractive power of the human eye at different diameters at a constant value so as to achieve better visual quality. The refractive power provided by the negative diopter lens of a spherical design decreases (absolute value increases) as the aperture diameter increases, which causes the human eye to form hyperopic defocus and facilitates increase of the axial length of the human eye, thereby accelerating the development of myopia. Existing PIOLs of an aspheric design maintain the refractive power of the human eye at different diameters at a constant value, which compared with the curvature of the retina, would also form hyperopic defocus and thus accelerate the development of myopia.
Therefore, there is a particular need for an intraocular lens to solve the above-mentioned existing problem.